1. Field of the Invention.
This invention relates to a composition of matter useful as a fine grain refractory material, especially in applications for contact with molten metals in such particular devices as nozzles and gates, and to processes for producing the same. The refractory ceramics of the present invention exhibit excellent wear, thermal shock, and corrosion resistance in high temperature applications. 2. The State of the Art.
The manufacture of iron and nickel alloys, particularly specialty steels, is a highly competitive industry. Procedures for reducing manufacturing costs and down times can lead to dramatic competitive advantages.
One general category of manufacturing is casting. This method typically involves the pouring and metering of a molten metal or alloy from a ladle into a tundish, which is a large holding area. Integral with the bottom of the tundish are a number of flow channels, or nozzles, through which the molten metal is carefully metered as it pours through to the casting mold. (Metering is intrinsically accomplished by having a molten metal under an essentially constant head flow through a constant diameter opening.) The flow rate through a single nozzle can be quite high, for example, ten tons of steel per minute. Accordingly, the ceramic refractories used in steel and other molten metal industries should not only possess high thermal shock, wear, and corrosion resistance, but should also prevent heat loss, oxidation of the molten stream, and uncontrolled flow. The refractory ceramic products must also be reliable and economical due to the cost competitiveness of the commodity metals industries. In particular, economical advantages should inure to the refractory products not only by providing such products at a reasonable cost, but the reliability and service life of such products dramatically effects the integration of the metal processing operation, such as down time versus continuous operating time, and thereby the economics of the processing operation.
Currently, the principal problems associated with refractory ceramic materials are wear and reliability. For example, a nozzle's key function is to precisely channel and meter molten materials. The nozzle wears as molten metal passes through it, and thus the bore changes dimension and shortens as the ceramic material is eroded from the bore and nozzle ends. When the wear progresses to a point such that the molten metal flow can no longer be metered with the requisite accuracy, the process is stopped and the nozzles are replaced. The need to frequently replace these key refractory components severely limits the efficiency of the processing operation.
A typical microstructure for a common tundish nozzle is shown in FIG. 1. The microstructure is characterized as a coarse agglomerate of "grog" grains which are cemented together with an inorganic, refractory cement; silica is commonly used as the refractory cement material. The porosity of such typical structures generally ranges from 13% to 30%, depending on the particular refractory application. The coarse grains and high porosity present in such a structure is due, in large part, to traditional engineering philosophies designed to protect the ceramic piece against thermal shock (typically the cool refractory is suddenly contacted with approximately 1600.degree. C. molten metal). Unfortunately, this porosity results in poor wear and corrosion resistance; with increasing porosity, the number and area of contacts at which the large grog grains are cemented together is greatly decreased. This results in severe erosion as the molten metal, which is flowing at a high rate and under a large dynamic head, picks off grains from the nozzle bore surface. The low strength of such typical materials, inherent from their microstructure, also greatly contributes to chipping and cracking of the refractory ceramic prior to use. If such defects are not noted, this results in premature refractory failure, thereby further adding great expense and inefficiency to the steel making process. The penetration depth into and wetting of the molten metal onto the bore wall is also greatly increased with increasing porosity, often resulting in degradative chemical reactions between the refractory grains and the molten metal. For example, alumina-based refractories are subject to mullite formation when contacted with manganese/silicon low oxidative steels. As is commonly practiced in the steel industry, some steels are "killed" with aluminum to bind oxygen present in the steel. When such aluminum-killed steels, in which the steel contains precipitates of aluminum oxide, are poured through nozzles, the precipitates wet and cling to the nozzle bore, thereby clogging the bore and corroding the refractory material. The wetting and intercalating of the precipitates into the coarse grains can be temporarily delayed through pitch bonding (that is, the application to the bore of a non-wetting material), but such coatings are equally susceptible to degradation. Corrosion can also occur when the precipitates adhere to the bore wall, thereby causing restricted flow. Further, when the nozzle wears unevenly, the bore must be cleared with an oxygen lance; however, extremely high corrosion rates at the lanced section of the bore result, further leading to a shortened useful life. Corrosion resistance is generally minimized by the proper material selection; unfortunately, this selection is, by and large, an empirical art.
Ceramic refractory materials, such as used in the metal industries, are typically coarse grain, cement bonded, parts, and are typically composed of one or combinations of the following materials: alumina, magnesia, zirconia, zircon, mullite, silicon carbide, silicon nitride, pitch, graphite, and clay. In general, the chemical composition desired is determined through empirical methods to achieve the desired optimum properties regarding corrosion resistance, nozzle bore properties, wear, etc. For example, to combat nozzle clogging, Kagimi et al., U.S. Pat. No. 4,510,191, proposes a coarse grain bonded alumina refractory in which graphite is incorporated into the microstructure to create a slippery nozzle bore. That patent describes that while conventional zirconia and zircon refractories are resistant to erosion, the alumina precipitates nevertheless wet the ceramic and result in rapid clogging. A popular composition for many applications includes alumina-graphite refractories which are clay bonded, but again these are not very thermal shock resistant and are fragile and easily corroded, although they have a reduced tendency to clog due to the addition of graphite.
Typical coarse grain refractories used in the molten metal industries, such as for the channeling of metals, are described in Holt et al., U.S. Pat. Nos. 3,972,722 and 4,253,590, which describe two coarse grain compositions including alumina-zircon and magnesia, respectively. In these two patents, Holt et al. describe the use and benefits of a size distribution composed of 30-80% grog (4-70 mesh), 6-20% zircon (-74 microns) and 4-45% fines (-44 microns). Gotoh et al., in U.S. Pat. No. 4,646,950, describe the use of 30-60% coarse and medium particles (1-4 millimeters and 0.3-1.0 millimeters, respectively) and the balance composed of -0.3 millimeter fines, for alumina-zirconia-titania sliding nozzle plates.
Fine-grain zirconia and zircon ceramics have been recently described by several people, in the context of toughness for applications at less than about 1000.degree. C.; accordingly, these descriptions provide little motivation to use such ceramics in the molten metal processing industry. Garvie et al. U.S. Pat. Nos. 4,279,655 and 4,067,745, discuss magnesium oxide and calcium oxide doped zirconia ceramics as PSZ (partially stabilized zirconia) ceramics for high strength and toughness at temperatures below 500.degree. C. Garvie et al. also discuss a calcium oxide-zirconia composite for low temperature refractory application such as metal cutting and metal drawing. Tsukuma et al., in U.S. Pat. No. 4,587,255, discuss a fine grained yttria stabilized tetragonal zirconia polycrystal (TZP) ceramic that exhibits high strength and toughness for cool applications at temperatures less than about 1000.degree. C. However, these materials are not very thermal shock resistant, nor are these fine-grain materials specified for a high temperature molten metal contact environment. Also, these TZP materials are expensive due to the fine nature of the powders and the cost of the yttria stabilizing agent. The processing of the PSZ materials described above requires special and expensive processing equipment to enable sintering in the 1700.degree. C. to 1950.degree. C. temperature range.
More recently, Garvie, in U.S. Pat. No. 4,579,829, describes zircon-zirconia refractories, wherein the zircon has a purity of at least 97% and the zirconia has a purity of at least 99%. In this patent, Garvie describes that dissociated zircon (DZ) has acceptable refractory characteristics, but cannot be processed in an aqueous environment (e.g., processed such as by slip casting) and thus Garvie utilizes a zircon-zirconia refractory because it is a less expensive and more easily processable substitute for a DZ ceramic.